test: mark stringbytes-external-max flaky on AIX

[sxa]

Another one in the set of tests marked flaky on AIX under #60565 - this is also exhibiting intermittent failing behaviour and is causing issues for PRs such as #60962 (comment) (which was on v24 so I'll backport to there once this is in)

Verifying it's level of flakiness in https://ci.nodejs.org/job/node-stress-single-test/628/ (Seems in the same ballpark as the others)

Screenshot from https://ci.nodejs.org/job/node-test-commit-aix/60068/nodes=aix72-ppc64/testReport/addons/tringbytes-external-exceed-max/ :

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70437/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.54%. Comparing base (40c57ed) to head (13e357e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60995      +/-   ##
==========================================
+ Coverage   88.53%   88.54%   +0.01%     
==========================================
  Files         703      703              
  Lines      208432   208432              
  Branches    40195    40196       +1     
==========================================
+ Hits       184526   184555      +29     
+ Misses      15925    15890      -35     
- Partials     7981     7987       +6     

see 42 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70442/